In recent years, lightweight transparent polymer films have been extensively used in the field of displays, as can be seen in the example of cellular phone displays. In addition, polymer films are considered to be one of the indispensable constituent elements of conventional paper displays.
Currently known films, which constitute one of the most suitable technological fields for polymer materials, include various films of polyethylene, polypropylene, polyethylene terephthalate, and other crystalline polymeric films as well as films of polycarbonate, polymethylmethacrylate, and other amorphous polymers. All of the above-mentioned materials are thermoplastic polymers that readily lend themselves to use in the production of various types of films by adjusting their molecular weight and molecular weight distribution.
Many of the currently commercially available transparent polymer films are made of thermoplastic polymers and are manufactured, for instance, by calendering molten thermoplastic polymers or by means of extrusion molding them using a T-die. In addition, transparent films can be produced by biaxially stretching crystalline polymers.
Films made from thermoplastic polymers are known to be prone to molecular chain orientation that results from the method of their manufacturing.
In the case of transparent films, such orientation phenomena present a particularly serious problem. This is because birefringence that results from light passing through a transparent film whose molecular chains have been oriented causes polarization of light. Therefore, in terms of their practical application as optical materials, thermoplastic polymer-based transparent films have been viewed as problematic.
Although the orientation of thermoplastic polymers is caused by the stress applied thereto in the molten state, the application of a certain amount of stress during molding in the molten state is impossible to avoid. Therefore, in order to suppress the orientation of the film, it is a good idea to select a molding treatment that does not produce stress, such as, for instance, cast molding with the use of a solvent. However, cast molding requires that a casting solution be prepared by dissolving a mixture of a thermoplastic polymer with various additives such as a UV absorption agent, antioxidant, etc.
Known in the art are thermoplastic amorphous polymers of high thermal resistivity, such as polysulfone. However, a light-absorption band may exist in polysulfones approximately up to 400 nm, and from this point of view, this polymer cannot be easily obtained with high optical transmissivity.
On the other hand, in the case of thermosetting resins, molding does not need application of pressure since liquid monomers or low-molecular-weight prepolymers are subject to cross-linking and increase in molecular weight. This prevents orientation of polymer molecular chains.
Normally, films made from thermosetting resins are supplied in shapes that can be maintained by being supported on appropriate substrates, and in many cases is it is quite difficult to obtain a self-supporting film that is not intended for being maintained on a substrate.
Polysiloxanes can be produced in the form of transparent films that possess superior thermal stability, UV ray stability, anti-oxidation stability, etc. without addition of heat-resistance stabilizers, UV absorbants, anti-oxidants, or the like. However, they are intended for being supported by substrates, and self-supporting polysiloxane films having sufficient physical properties are still absent on the market.